In the unsophisticated form it's that atheism leads to immorality (like the caller on a radio talk show Dawkins was on in the US, who said that if he thought there wasn't a God he probably would murder his neighbour). This isn't really worth engaging with, because it's not an argument that atheism is false.
In the more sophisticated form the argument is that atheism, if true, necessarily means that morality is an arbitrary personal opinion. But we strongly feel that some things are just wrong regardless of anyone's opinion (in this argument, rape and the Nazis are the canonical examples of things that are just wrong). This contradicts atheism, so atheism must be false.
The latter form of the argument came up recently in an interview that Premier Christian Radio's Justin Brierley did with Richard Dawkins after a debate Dawkins was in. Brierley wrote a piece about it on the UCCF's BeThinking.org site. robhu has posted about it on his journal, and has a poll on what people think about the morality of very bad things. Some lively discussion has ensued there.
Edit: but unfortunately Rob deleted his LiveJournal a while back. Here's what I said:
Although this "OMG you atheists can't claim Hilter/rape is wrong" argument seems popular among evangelicals at the moment, I'm not sure what the argument against atheism actually is.I also responded to one of Rob's objections:
Most atheists demonstrably do claim that Hilter and rape are wrong, so the argument seems to be that such claims aren't well-founded if atheism is true, so that atheism is inconsistent.
There are atheists who are moral realists, although I've not checked whether their arguments are any good. Still, there are some serious names in that Wikipedia article (and Ayn Rand), so I'd be reluctant to conclude that they're inconsistent without looking into it.
Even if atheism is inconsistent with the existence of moral absolutes (note: I originally wrote "moral realism" here, but that's not the topic), in the absence of evidence that there is such a thing as objective morality, this sort of argument does not seem to demonstrate that atheism is false, merely that if atheism is true, the universe is not as we'd like it to be (in the sense that we'd like it if there were moral absolutes). The objection to atheism on these grounds seems to be wishful thinking.
Personally, while I think there could be beings who thought that rape and Hitler were not wrong, most of us are not such beings and (crucially) do not want ourselves or others to become such beings. That is, arguments that these things are wrong can be recognised by most humans, but aren't guaranteed by the universe/God/whatever.
You suggest that it's wishful thinking if our deepest sense of what is true does not match up with your criteria for objective proof of that sense. Which I take to be a position where you say you have some strong inner sense that say the holocaust is wrong (even if everyone who disagrees is exterminated or brainwashed to believe otherwise), but because that doesn't match with the worldview you have (that is there is no God, and so no objective morality) you say that it's just wishful thinking. For those of us have the worldview that God is real, it makes a great of deal of sense.
So, if I understand what you're saying, "our" is moral absolutists, "your" is me, right? So you're saying I, pw201, have a strong sense that Bad Stuff is wrong (which is true), further, that I think it'll be wrong even if everyone else disagrees (which is true). But I also think such a position (i.e. my own) is wishful thinking, which means I look a bit silly.
But in fact what I think is wishful thinking is the objection to atheism on the grounds that it would mean there are no moral absolutes, because the only grounds for that objection I'm aware of at the moment is that the objector would like it if there were moral absolutes, not that there actually are moral absolutes.
You might be saying that what I've said about Bad Stuff two paragraphs ago means I do accept that there are moral absolutes, but in fact all I've said is what I think, not that God/the Platonic Form of Moralty agrees with me.